The long-term objective of this project is the elucidation of basic principles about CD4+ memory T cells that can be applied to the design of more effective vaccines. CD4+ memory T cells are part of the host's immune system that responds more quickly and strongly to the second exposure to a foreign antigen than it did to the first. During the last funding period we developed a sensitive new technique to track polyclonal peptide major histocompatibility complex class II molecule (pMHCII)-specific CD4+ T cells in normal mice and used it to study immune memory induced by bacterial infection. We found that acute infection with an attenuated strain of Listeria monocytogenes (LM) generated T-bet+ IFN-3-producing Th1 cells, a novel population of Tbet- cells of unknown function that depended on an unknown signal from B cells, and follicular helper cells (Tfh) that depended on pMHCII presentation by B cells. The Th1 cells and T-bet- cells became memory cells but the Tfh cells did not. In contrast, persistent infection with Salmonella typhimurium (ST) induced only T-bet+ cells, which were more stable and contained more multifunctional cells capable of making IFN- 3 and TNF than memory cells induced by acute LM infection. The goal of the current project is to understand how the CD4+ memory T cell subsets induced by these infections are generated and provide protective immunity. LM infection will be studied to determine how the T-bet- memory cells are generated, whether they are what have historically been called central memory cells, and how they help B cells and CD8+ T cells during the secondary immune response. This research is significant because it could give clues about how to improve vaccines against acute infections. ST infection will be studied with the goal of identifying which variety of CD4+ T cell is needed to control infections that threaten the lives of AIDS patients, determine how these cells are generated and provide protection, and ascertain whether they can ever become truly long-lived memory cells. This research is significant because it could fill the knowledge gap that currently prevents creation of more effective T cell-mediated vaccines. This research is innovative because it will employ novel technology to track the endogenous polyclonal CD4+ T cell response during infection, and because it is based on the new ideas that anatomically confined chronic pMHCII presentation without the involvement of B cells is the best way to generate multifunctional CD4+ T cells capable of controlling intracellular bacterial infections.